The invention relates to magnetic tape capable of optically recording servo signals for tracking. More particularly, it relates to magnetic tape capable of optically recording servo signals for tracking while exhibiting excellent tape runnability and other mechanical properties.
Generally, magnetic tape has a low recording density due to its low track density. Serpentine type magnetic tape in particular typically has a low recording density. On the other hand, helical scan type magnetic tape, which uses a servo tracking system called automatic track finding (ATF), possesses a higher track density than serpentine type magnetic tape.
Servo tracking systems proposed for serpentine type magnetic tape include an embedded servo system, in which servo signals are written on the same track as the data track on the magnetic recording surface, and a system in which a track exclusively for servo signals is provided on the magnetic recording surface. For example, Japanese Patent Publication No. 82626/95 discloses a servo control system where the pitch of the data tracks is as small as several tens of microns. In this system, a dedicated track for servo information is provided on the magnetic recording surface and a plurality of servo reproduction heads are used for reading the servo signals. According to this technique, however, the number of servo reproduction heads must be increased as the number of tracks increases. This system and other conventional servo tracking systems use the same side of the magnetic tape for servo tracking as is used for data recording, which means that the data recording area is reduced accordingly. This disadvantage is conspicuous in the servo tracking system of Japanese Patent Publn. No. 82626/95 when the track density is about 30 tracks per mm or more.
This disadvantage can be overcome by locating the servo tracking system on the side of the tape opposite the recording (magnetic) side. Such a system is described in co-owned co-pending application USSN 09/191,321, which is hereby incorporated herein by reference. The system described in USSN 09/191,321 utilizes a pattern containing a dye or other material which changes color upon being contacted with a light source, thereby permitting servo tracking of the tape.
Magnetic tapes often have a coating or layer disposed on the side of the tape opposite the recording side. Such coatings are called xe2x80x9cbackcoatingxe2x80x9d layers. Backcoating layers are designed to improve certain mechanical properties of the tape. For example, the presence of an appropriate backcoating can improve the tape runnability (e.g., the stability of the tape as it xe2x80x9crunsxe2x80x9d past reading or recording heads) or its durability. Backcoating layers often contain a binder resin and an inorganic pigment, such as carbon black. See, for example, JP 9297914; JP 6139549; KR 9406847 and U.S. 4,578,311. USSN 09/191,321, incorporated by reference above, describes adding a backcoating to the tape in addition to the servo tracking color changing coating.
It is an object of the present invention to provide a magnetic tape having a multilayer backcoating system which has excellent mechanical properties and which permits a servo tracking pattern to be etched directly into the backcoating.
The present invention is a magnetic tape including a recording surface on one side and a non-recording surface on the side opposite the recording surface. The recording surface has a magnetic layer disposed thereon. The non-recording surface has disposed thereon a novel, multilayer backcoating system including at least two discrete layers containing inorganic particles in which the particles in the outermost layer are different from (i.e.,larger than) than the particles in the inner layer(s). The larger particles in the outer layer are of a size sufficient to provide improved tape runnability and conductivity, and reduced reflectance. The smaller particles in the inner layer or layers are of a size sufficient to provide improved servo cutting quality to the tape.
The backcoating system on the tape of the present invention has a regular pattern for servo tracking disposed along the longitudinal direction of the tape. The servo tracking pattern may include, for example, a series of discrete marks, such as pits or depressions, in the backcoating. The pattern also may comprise continuous depressed lines or grooves. The pattern can be formed, for example, by etching, cutting, engraving or burning the backcoating, e.g., mechanically or using a laser. The process of forming the servo tracking pattern is referred to herein as xe2x80x9cservo cutting.xe2x80x9d The marks thus formed have optical properties which are different from the non-patterned region of the backcoating, and can be distinguished by an optical tracking device. For example, a pattern formed by laser etching may comprise a series of pits or depressions in the backcoating. When a light source is directed onto the backcoating, the pattern of depressions will absorb or reflect light differently than the surrounding non-patterned region of the backcoating. These differences can be detected by an optical detector, and processed by a signal processor to operate the servo system which keeps the tape property aligned. The pattern permits servo tracking of the tape as it runs thereby improving the track density of the tape.
The present invention provides a multilayer backcoat system which has excellent mechanical properties and also permits high quality servo cutting. The present invention accomplishes both purposes by utilizing a multilayer backcoating system in which the outer layer, which in operation is in contact with the tape rollers, has a surface roughness sufficient to provide good mechanical properties; and an underlying or inner layer or layers having less surface roughness, that is, having a smoother surface, thereby providing better servo pattern cutting.
In order to provide high quality servo cutting and tape runnability, the backcoating preferably has a uniform smooth surface. However, smooth coatings can result in deterioration of certain mechanical performance of the tape. For example, tapes having a very smooth backcoating may exhibit poor lateral tape motion, high friction between the tape and the tape rollers, and poor tape modulation and stacking. Rougher backcoating surfaces provide better electrical conductability, better heat conductability and improved mechanical properties, but the quality of the servo cutting is reduced.
The roughness or smoothness of the surface is a function of the size of the inorganic particles used in the backcoating. Larger particles provide a rougher surface, whereas smaller particles provide a smoother surface. Thus, in the present invention, the outermost backcoating layer includes a coating or film having larger inorganic particles (e.g., having an average size greater than about 50 nm) and the inner layer comprises a coating or film having smaller inorganic particles (e.g., having an average size less than about 40 nm). There may be more than one inner layer, if desired.
The present invention provides a magnetic tape which furnishes servo information without reducing the data area, a magnetic tape which furnishes information for servo tracking while maintaining a high S/N ratio and excellent tape runnability, a magnetic tape which furnishes servo information without impairing the mechanical properties of the tape provided by the backcoating layer, a magnetic tape having an increased track density, and a magnetic tape having a high recording capacity.